Redesigning TOR Kinase to Explore the Structural Basis for TORC1 and TORC2 Assembly

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The Target of Rapamycin (TOR) pathway regulates cell growth through distinct TOR Complex 1 (TORC1) and TOR Complex 2 (TORC2).
• Mammalian cells have one mTOR protein, while yeast has two, Tor1 and Tor2, with differential complex assembly, offering a unique model for studying specificity.

Purpose of the Study:

• To identify structural determinants governing the specific assembly of Tor1 and Tor2 proteins into TORC1 and TORC2 in yeast.
• To understand the molecular basis of TOR complex specificity.

Main Methods:

• Utilized the yeast bifurcation of Tor1 and Tor2 to investigate complex assembly.
• Employed protein engineering by creating chimeric proteins swapping the N-terminal Major Assembly Specificity (MAS) domain between Tor1 and Tor2.

Main Results:

• Identified a ~500 amino acid N-terminal region in Tor2, the MAS domain, sufficient to confer TORC2 activity when introduced into Tor1.
• Demonstrated that the Tor1 MAS domain, when transferred to Tor2, confers stable association with TORC1-specific components.
• Observed that the chimeric Tor2 protein with the Tor1 MAS domain retained TORC2 interactions, suggesting broader determinants for complex specificity.

Conclusions:

• The MAS domain is a key determinant of TOR complex specificity in yeast.
• Both MAS-dependent and potentially other regions contribute to the precise assembly and stability of TORC1 and TORC2.
• Findings align with ultrastructural studies highlighting the MAS domain's role in quaternary interactions for TOR complex formation.